Psyche=mind; Soma=sleep. {more}

Basically, it means that it's coming from your mind without your being consciously aware of it. It can be meaningful, but it can also mean that the doctor is writing off a symptom. (For instance, I was once hospitalized for a "psychosomatic" bladder infection: by that time the very real infection had moved to my kidneys and almost killed me! The doctor had missed the infection in tests, but it was there.)